Ionization gauge for the measurement of low pressures



Apnl 16, 1963 A. REDHEAD 25,369

IONIZATION GAUGE FOR THE MEASUREMENT OF LOW PRESSURES Original Filed May'7, 1957 1 1 11/ l I I I 1.! I41 111 Illl'lll'll' [I] 111'!!! 1 27 mamllllllll'f 3 13 37; I9 17 J ifu P'ATENT AGENT United States PatentOfifice Re. 25,369 Reissued Apr. 16, 1963 25,369 IONIZATION GAUGE FORTHE MEASUREMENT OF LOW PRESSURES Paul Aveling Redhead, Ottawa, Ontario,Canada, assignor to National Research Couucii, Gttawa, Untario, Canada,a body corporate of Canada Original No. 2,937,295, dated May 17, 1964Ser. No. 657,624, May 7, 1957. Application for reissue Sept. 7, 1960,Ser. No. 54,554

3 Claims. (Cl. 324-46) Matter enclosed in heavy brackets 1 appears inthe original patent but forms no part of this reissue specification;matter printed in italics indicates the additions made by reissue.

This invention relates to a vacuum gauge of the ionization type andparticularly concerns ionization vacuum gauges employing a magneticfield throughout an ionizing space.

Conventional ionization vacuum gauges are constructed somewhatresembling a triode vacuum tube employing a filamentary cathode, anaccelerating grid, and a cylindric plate electrode, electrons emittedfrom the filament moving at high velocity toward and through thepositive grid colliding with ionizing molecules of a gas filling thetube. The number of ions produced by kinetic collisions per unit time isassumed proportional to the density of the gas, all other factorsremaining constant, and hence proportional to pressure. A currentobtained by collecting the ions so produced upon the negatively chargedelectrode is measured to provide an indication of the degree of vacuum.Actually it is the ratio of ion current measured to the electron currentto the grid of such devices that may correctly be said to represent thepressure. Owing to fundamental limitations the gauge current indicationsare not valid for pressures below about mm. of mercury.

In order to increase by a large factor the possibility that movingelectrons liberated from a source will yield a useful ion population bymaintaining a gas discharge in the chamber, a unidirectional magneticfield has been applied of such shape and intensity with respect to theelectrodes as to cause spiral paths to be taken by the electrons.Orbitally spiralling ions have their cathode-toanode track lengthsgreatly increased as compared with the radial distance between a sourceof initiating electrons and an ion collector. A device operating on thisprinciple is described in United States Patent 2,197,079 to Penning,using a cold cathode with a ring or cylindric anode. With devices ofthis type using nitrogen for example, pressures as low as 10' mm. Hg areobservable. However the establishment of an avalanche discharge becomesimpossible at the lower limits of pressure since the electron source isinefiicient when the mean free path distances between gas moleculesbecome a large fraction of the anode-cathode distance. Moreover, onlythe total current of the space discharge is measurable in such device,precluding the exclusion of electron current and photon currents fromthe measured total. At the pressure indicated the ion current becomes asmall fraction of the total measured.

The range of pressures extending between about 10* and 10 mm. Hg arepresently measurable by a special type of ionization gauge known as aBayard-Alpert gauge as described in an article New Developments in theProduction and Measurement of Ultra-high Vacuum, D. Alpert, J.A.P. 24,860, 1953. A gauge of the latter type has been shown to also act as anionization pump in attaining the lowest measurable vacuum. The gaugeemploys a heated filament externally of a positive accelerating grid,and both ion current to a central anode wire and electron current to theaccelerating grid are measured to establish the pressure.

Hitherto all existing ionization gauge measurements have included errorcurrent due to X-ray effects which confuse the lowest measurable ioncurrent indications observable. The ionizing electrons in the gaugeproduce X-rays on collision with metal anode, grid, or wall structure,which in turn cause photoelectric emission of photons from the positiveion collector and from the envelope walls. Photoelectric currents due tosuch excitation by X-rays are indistinguishable from the net ion currentat the positive ion collector. Therefore, depending on the geometry ofthe tube, the lowest measurable ion collector current is predominantlyphoto-electron emission current at pressures of the order of 10* mm. Hgand below. The Bayard-Alpert design, for example as described andclaimed in United States Patent 2,605,431 to Bayard, has lowered thislimit to about 10' mm. Hg by a design making the anode slender tointercept a very small solid angle, and insulating the ion collectorlead to minimize charge leakage from glass walls.

The present invention seeks to avoid the difliculties of X-ray emissionand inefiicient ionization mechanism which have hitherto been limitingfactors, by relying upon cold emission of electrons, spirally lengthenedion and electron paths, and isolating the positive ion collector fromother electrodes. Embodiments of the invention are characterized in thatthe function of electron emission is reserved solely to an auxiliarycold electrode while the collection of positive ions within anionization space is solely performed by a separate electrode. These arereferred to hereinafter, respectively, as the auxiliary cathode and themain cathode. According to the invention a central anode of rod shape iscoaxially related within an enclosing cylindric shell main cathodehaving apertures in its end walls, and disc-like auxiliary cathodeshaving apertures through which the high voltage anode extends locatedadjacently to and outside of the ends of the main cathode, the coldemission of electrons from the edges of the auxiliary cathode aperturesbeing due to field emission under high potential gradients.

The anode is given a high positive voltage with respect to the auxiliarycathode which is maintained at ground potential while the positive ioncurrent reaching the main cathode is measured by a micro-current-sensingdevice connected in series between it and ground. A strong magneticfield is applied along the axis of the device in the direction of theanode, which causes the electron paths to become spirals whose lengthsare on the average many hundreds of times greater than the electron pathlengths in the absence of the magnetic field. By this increase of pathlength and by the improved mechanism of electron emission a highionization efiiciency is produced and a high sensitivity is thereby alsodeveloped. Hence a measurable flow of ion current is realized even withpressures well below 10* mm. Hg.

Embodiments of the invention herein described may be the betterunderstood by reference to the accompanying figures of drawing, wherein:

FIGURE 1 is a longitudinal axial section showing the arrangement ofelectrodes;

FIGURE 2. is a plan view of the apparatus of FIGURE 1 partly in sectionalong line 2-2;

FIGURE 3 is a modification of the electrode structure of FIGURE 2showing alternate auxiliary electrode structure; and

FIGURE 4 is an illustration of a modified cathode.

The apparatus shown in FIGURE 1 consists essentially of threeelectrodes, namely, straight anode wire 10 c0- axial with thecylindrical cathode shell 11 having end plates 12 and 13 with largecentral holes 14 and 15, and two auxiliary cathodes 16 and 17 which areflat discs closely spaced from the end plates of the main cathode, andhaving small central holes 18 and 19 coaxial with the anode. Highpositive potential of the order of 5 to kilovolts for example is appliedto the anode with respect to the grounded cathode, and a uni-directionalmagnetic field is applied along the axis of the gauge, supplied as bypole pieces 21. A small number of electrons are emitted from the edge ofthe holes 18 and 19 by field emission. These electrons are drawn intothe cathode cylinder 11 by the electric field adjacent the emissionregion, wherein the equipotentials are surfaces of revolution about theanode. A certain proportion of the total electrons liberated move intothe cylindrical chamber 22 where under the influence of radial electricand axial magnetic fields they orbit about the anode wire and driftaxially, eventually being collected upon the anode wire. By virture ofthe coaxial relationship, the gradient between the main cathode and theanode as observed in a plane passed normally through the middle of thecollector is logarithmic. The inward radial acceleration of electrons isrelatively small until they have orbited to a relatively very shortdistance from the anode. Both the initial electrons and those liberatedfrom gas molecules upon ionization contribute to the production ofpositive ions by collision with the gas molecules within the cathodecylinder space 22 during their spiral transits towards the anode. Thesepositive ions move outwards in generally curved paths and finallyreachthe main cathode 11 to which they give up their charges. The rateof charge transport is detected and indicated by any suitable device 23connected between the cathode and ground capable of reading very weakcurrents.

'It will be apparent that in the construction described the currentpassing to the electron-emitters 17 and 18 is entirely separate from theion-collector current, so that the concept of a ratio of currents as hasheretofore required to be considered in ionization gauges does notpertain to the device according tothe invention.

Photo-electron current resulting from Xrays Within the collector 11,generated by electrons striking the anode wire, are reduced veryconsiderably in comparison with prior art apparatus by the practice ofthe invention.

In contrast with prior art efforts to mitigate the X-ray radiationreceived by a positive-ion current collector, by geometryof the gaugeelements, the invention provides a magnetic field by which the netphotoelectron flow from the main cathode is caused to be very small.X-ray radiation from the anode rod impinges upon the main cathodefreely, causing relatively copious numbers of photoelectrons to beemitted as a result of this energy. However the great majority of theseare ejected at low energies and hence make orbits of small radius intospace 22, returning to the surface. The net liberation of photoelectronsfrom the main cathode 11 does not compare with the positive ion currentthereto until a gas pressure below about 10 mm. Hg isattained, due tothe greatly increased ionization efiiciency of the apparatus.

In FIGURE 3 modified field-emission cathodes 18 and 19 suitable for usein devices measuring the lowest pressures are provided with cylindricbaffles 35, 36 coaxial with the anode, whose functions are partly tointercept and attenuate such Xrays as emanate from the anode withsubstantially radial directions, and partly to shield the main cathodeend discs 12 and 13. In this embodiment the inner radius of each of thediscs 12 and 13 is made relatively small to provide a small clearance ofabout one mm. from the outer face of the baffle. The auxiliary cathodesare also preferably formed of a single sheet of metal bent to form abox-like shield to reduce external influences on the main cathode whichwould otherwise impair the accuracy of current measurements.

Referring to FIGURES 1 and 2, the apparatus includes an envelope 27 of amaterial which is impervious to gas but pervious to magnetic field,having two flat opposed faces adapted to lie between the pole pieces 20,

21, a press 28 in one end, a tubular pumping connection 24, and an anodelead-supporting tubulation 25 having a re-entrant portion 26. It hasbeen found that by passing the anode connection through the envelopewall at a location remote from the press 28, and preferably bysupporting it on a re-entrantly folded tubulation 26, leakage currenterrors to the ion collector lead-in are minimized.

Cathode 11 is preferably formed of a screen or gauze so that flow of gasmolecules to the interior space 22 is not impeded. The end walls 12 and13 are preferably of mesh form or pierced, and the openings 14 and 15should be at least one collector radius distance in diameter. Thestructure is supported on a lead-in wire by a rigid bar 29 spot weldedto stiff wires 30, 3-1 attached to the ends of the cathode. The anode isa slender rod, preferably of tungsten, of about 0.040 inch diameter,under slight tension between the ends 32, 33 of a U- shaped wire 37,which is itself spot-welded to anode lead-in 34. The cold-cathode discs16, 17 are similarly supported on a lead-in 38 secured in press 28. Bythe construction described a rigid assembly is realized with minimumleakage currents along the envelope inner surfaces between electrodes.

The anode rod diameter has been found to have an influence on thestriking voltage required to start up the ionization discharge.Extremely fine anode wires exhibit a characteristic non-linearrelationship of striking voltage with respect to wire diameter, foranodes less than about one mm. diameter. Preferred sizes of anode arefractionally larger than one mm. diameter.

FIGURE 4 describes an alternative cathode disc construction wherebyminimum obstruction is offered to the axial movement of gas moleculesinto chamber 22. A ring 40' having an aperture 18 whose inner edge ismade sharp for field emission is supported by three radial arms.Numerous other modifications will suggest themselves, and the use ofgauze instead of radial arms is envisaged. A point or spike cathodepresented adjacent the anode may be substituted in lieu of the disc orring structure described. 'Cathodes 16 and 17 are preferably made of anyreasonably refractive metal.

Ion gauges have been shown heretofore to serve effectively as pumps forevacuating a system at pressures which are already low, for examplebelow about 10" mm. Hg. In general the pumping speed of a gauge whereinthe outgassing is strictly by electrical capture and adsorption of ions,is dependent on the area of negatively charged surfaces and on theionization efiiciency expressed as the net number of ions formed persecond within the space. The time taken to reduce the pressure of aclosed system to the l/e value of the pressure difference between anactual pressure p and the ultimate pressure p is directly proportionalto the volume and inversely proportional to the pumping speed. It isdirectly apparent that any apparatus capable of ionizing gas moleculesand entrapping ions at a high rate will have a high pumping speed.Experiments have amply verified that gauges constructed according to theinvention are highly efiicient pumps and are relatively insensitive tothe strength of initiating electron current liberated from the cathode.

In an actual embodiment according to FIGURE 3, the interior diameter ofthe main cathode was 30 mm., the axial length 15 mm., the field strength2000 gausses, and the applied voltage 5 kilovolts. The measured pumpingspeed at 10- mm. for dry air was of the order of 0.05 liter/sec.

While I have described embodiments of my invention in considerabledetail ll wish it to be understood that I am not limited to theparticular forms or applications which have been described since manychanges of omission, addition, and substitution can be made withoutdeparting from the broader aspects of the invention, as may be fairlyconstrued to lie within the scope of the appended claims.

I claim:

1. In an ionization vacuum gauge, a non-conductive enclosure adapted tocontain a sample of a rarefied gas, an insulatedly supported cathodeelectrode adapted to be connected to a source of reference potentialcomprising an annular disc having a small bore therein and a coaxialtubular shell integral therewith surrounding the bore, means to supporta filamentary anode adapted to have a source of high positive voltageconnected thereto coaxially of the disc and extending beyond the tubularshell, an insulatedly supported closed cylindric shell ion collectorhaving apertured end walls coaxial of the anode and separate from andspaced from the cathode, said tubular shell etxending within thecollector and having a diameter substantially less than the diameter ofthe aperture of said end walls, and means to impress a magnetic fieldthrough the collector directed along the axis of said cylindric shell.

2. An ionization vacuum gauge comprising a hollow main cathode electrodeand an anode rod electrode defining an ionization zone therebetween,means to apply a magnetic field throughout said zone parallel with saidanode, spaced annular disc portions of said main cathode extendingtransversely of said magnetic field toward said anode and definingboundaries of said zone, auxiliary cathode electrodes disposed nearestsaid anode in shielding relation with said cathode portions, highvoltage supply means biasing said anode positive with respect to saidcathodes for impressing an electric field transversely of the magneticfield, the area of said cathode exposed to said ionization zone beinglarger than the exposed area of said auxiliary cathodes, and means forindicating positive ion current flowing to the main cathode.

3. A high vacuum gauge comprising an anode elongate rod electrode and ahollow cold cathode electrode, said electrodes defining an annularionization space therebetween, means for producing a magnetic fieldthrough said space in a direction parallel with said anode, spacedannular disc portions of said cathode extending toward said anodetransversely of said magnetic field, said portions defining opposedboundaries of said space and having marginal edges closest adjacent saidanode spaced uniformly therefrom, field emission cathode shields betweensaid anode and said marginal edges, bias means for biasing said anodepositive with respect to said cathodes whereby an electric field isimpressed through said space transversely of said magnetic field, andwhereby ionizing current flows to the cathode shields and ionizationcurrent flows to the cathode, and means connected between said cathodeand said bias means for indicating positive ion current flowing to thecathode.

4. A high vacuum gauge as claimed in claim 3 wherein the area of saidmain cathode bounding said space is larger than the area of said cathodeshields also bounding said space and said shields comprising annulardiscs coaxial with said anode.

5. A high vacuum gauge comprising an anode rod electrade and a hollowcold cathode electrode, said electrodes being bodies of revolutionhaving a common axis and defining an ionization zone therebetween, meansto apply a magnetic field throughout said zone parallel with said axis,end wall portions of said cathode extending toward said anode acrosssaid magnetic field, said portions having marginal edges spaceduniformly near said anode disc, cathode shield means mounted coaxiallybetween said marginal edges and said anode, electrical bias means forbiasing said anode positive with respect to said cathode and said shieldmeans, whereby a radial electric field is impressed through said spaceso that the potential gradient of the electric field adjacent saidmarginal edges is below field emission strength and whereby ionizationcurrent flows by field emission from said anode to said shield means andsubstantially only positive ions are collected by said cold cathode, andmeans for indicating positive ion current to said cathode.

6. A high vacuum gauge as claimed in claim 5 wherein the area of saidmain cathode in position to attract positive ions in said ionizationzone is larger than the area of said shield means in proximity to saidionization zone capable of attracting positive ions therefrom.

7. A high vacuum gauge comprising an anode rod electrode and a hollowcold cathode electrode surrounding said anode, said electrodes definingan ionization zone therebetween, means for creating a magnetic fieldextending through substantially all of said zone, a major portion ofsaid anode extending substantially parallel to said magnetic field,spaced portions of said cathode extending transversely of said magneticfield and toward the anode, said transverse portions being held inspaced relation from said anode, and constituting partly closing endwalls of said zone, field emission cathode shields between the opposedsurfaces of said transverse portions and the anode, means for providinga large difierence of electrical potential between said shield and saidanode, said shield being negative with respect to said anode, means formaintaining said cathode at about the some negative potential as theshield but electrically isolated from the shield, whereby ionizingcurrent flows between the shield and the anode and ionization currentflows to the cathode, and means for indicating positive ion currentflowing to the cathode.

8. A high vacuum gauge as claimed in claim 7 wherein the potentialgradient adjacent surface areas of said cath ode shield closest adjacentsaid anode is of sufi'icient magnitude to draw out electrons by fieldemission, and where in the potential gradient between said cathodesurfaces and said anode is below a magnitude required to draw electronsfrom said cathode transverse portions by field emission.

Conn July 31, 1956 Beck Dec. 18, 1956

